The updated constants include the Boltzmann constant (which relates temperature to energy), and
the Planck constant (which can relate mass to electromagnetic energy), the charge of the electron and the Avogadro constant (the quantity that defines one mole of a substance).

“The values of these four constants won’t change anymore,” said Peter Mohr, an employee and scientist at the National Institute of Standards and Technology (NIST). Mohr is also a member of the CODATA TGFC.

The new universal values will be fixed, he said, just as the speed of light is currently defined as an exact value. This in turn will allow scientists to focus on measurements that compare other important quantities to the constants.

“There are no dramatic changes. The Boltzmann constant is very consistent with earlier values,” said Mohr. “The temperature experts requested eight digits for the constant and the last digit happened to be 0,” he recounted—an amusing situation for metrologists since they can obtain the precision of eight significant digits by only having to use seven.

The volt will change as well, since the Planck constant will also help to define it in the revised SI. A volt based purely on the fundamental constants will be very slightly smaller, about 100 parts per billion, than the current scientific realization of the volt, established in 1990. So, the top-level metrology labs will have to recalibrate their high-precision voltage measurements.

That’s why the official rollout of the revised SI is slated for May 20, 2019, on World Metrology Day, to give metrologists time to adjust to the new values.

These constants underpin both science and commerce, ensuring fully uniform, convertible and precise measurements that scale smoothly from almost infinitesimal to enormous.

The kilogram has been defined since 1889, by a platinum-iridium cylinder stored in France, known as the International Prototype of the Kilogram – “Le Grand K .” Scientists from around the world have had to travel to France and compare their countries’ copies of the kilogram to the original in order to establish accurate mass measurements in their nations.

Temperature was related in terms of the “triple point” in a sealed glass cell of water. This is the temperature at which water, ice and water vapor exist in equilibrium. However, the water in these cells can contain chemical impurities that can shift the triple point temperature to inaccurate values. And measurements of temperatures higher or lower than the triple point of water leave more space for errors to occur.

Together with previously accepted constants, the updated values would redefine the SI’s seven base units, which include the kilogram (the unit of mass), the kelvin (the unit of temperature), and the ampere (the unit of electrical current).

The Planck constant has shifted downward by 15 parts per billion from its earlier value, due to new data collected since 2014. It can be used to define the kilogram, and using a fundamental constant for defining mass will solve many problems, said NIST’s David Newell, chair of the CODATA task group.

Mass must be measured over a very large scale, from an atom to a pharmaceutical to a skyscraper. “At the low end, you currently use one type of physics to determine mass; at the high end, you use another type of physics,” Newell said.